The present invention relates in general to a device for accurately aligning or positioning an object, and more particularly to the alignment of a mask and a wafer as used in photolithography in the manufacture of semiconductor devices.
In the manufacture of semiconductor devices, multiple processing steps are often required for producing a single wafer. The image of a mask is projected onto a wafer and the wafer is processed multiple times in fabricating a variety of layers to produce a semiconductor device, such as a microprocessor. In processing a wafer, accurate positioning and alignment of the wafer with a mask or reticle to be imaged thereon is often critical.
While there are many alignment sensors that use diffraction gratings, these alignment sensors tend to image a diffraction grating on a mask onto a diffraction grating on a wafer in order to detect alignment. One such alignment system is disclosed in U.S. Pat. No. 4,631,416 entitled xe2x80x9cWafer/Mask Alignment System Using Diffraction Gratingsxe2x80x9d issuing to Trutna, Jr. on Dec. 23, 1996, which is herein incorporated by reference. Therein disclosed is an alignment sensor and method with light diffracted from a mask grating to a wafer grating and back through the mask grating to produce diffraction orders. The intensity of the zero order is detected, and alignment occurs when the intensity of the zero order is at an extremum. A holographic phase grating is used on the mask too simplify W production of the grating. Another alignment system using gratings is disclosed in U.S. Pat. No. 4,848,911 entitled xe2x80x9cMethod for Aligning First and Second Objects, Relative to Each Other, and Apparatus for Practicing This Methodxe2x80x9d issuing to Uchida et al on Jul. 18, 1989, which is herein incorporated by reference. Therein disclosed is an apparatus and method for aligning a mask and a wafer. A first one-dimensional diffraction grating is formed on the mask and a second diffraction grating having a checkerboard like pattern, is formed on the wafer. Light beams diffracted from the first diffraction grating on the mask are transferred to the second diffraction grating on the wafer. The diffracted light beams from the second diffraction grating on the wafer are transferred to the first diffraction grating on the mask, and again diffracted by the first diffraction grating on the mask. The mask and wafer are precisely in line relative to each other in accordance with the intensity of the detected diffracted light beam. Another alignment system using diffraction gratings is disclosed in U.S. Pat. No. 5,100,234 entitled xe2x80x9cMethod and Apparatus for Aligning Two Objects, and Method and Apparatus for Providing A Desired Gap Between Two Objectsxe2x80x9d issuing to Ishibashi et al on Mar. 31, 1992, which is herein incorporated by reference. Therein disclosed is a first diffraction grating formed on a mask and a second diffraction grating formed on a wafer. Two light beams having slightly different frequencies interfere with each other and are diffracted by the first and second diffraction gratings. The diffracted light beams are combined into a detection light beam which has a phase shift representing the displacement between the wafer and the mask, or a phase shift representing the gap between the wafer and the mask. Another alignment system is disclosed in U.S. Pat. No. 5,151,754 entitled xe2x80x9cMethod And An Apparatus For Measuring A Displacement Between Two Objects And A Method and Apparatus For Measuring A Gap Distance Between Two Objectsxe2x80x9d issuing to Ishibashi et al on Nov. 29, 1992, which is herein incorporated by reference. Two objects, such as a mask and a wafer, each have at least one diffraction grating thereon. Two light beams of different frequencies are diffracted by the diffraction gratings. A light beam of a specific order is detected from each of the diffracted interference light beams and is converted into a beat signal. The displacement is obtained in accordance with the phase difference between these beat signals. Another alignment sensor or position detection apparatus is disclosed in U.S. Pat. No. 5,171,999 entitled xe2x80x9cAdjustable Beam And Interference Fringe Positionxe2x80x9d issuing to Komatsu et al on Dec. 15, 1992, herein incorporated by reference. Therein disclosed is a position detection apparatus or alignment sensor that has a diffraction grating on a substrate and an alignment optical system for illuminating the diffraction grating with a pair of coherent light beams having different frequencies and different directions. Yet another alignment sensor is disclosed in U.S. Pat. No. 5,559,601 entitled xe2x80x9cMask And Wafer Diffraction Grating Alignment System Wherein The Diffracted Light Beams Return Substantially Along An Incident Anglexe2x80x9d issuing to Gallatin et al on Sep. 24, 1996, which is herein incorporated by reference. Therein disclosed is a grating-grating interferometric wafer alignment system having a diffraction grating on a mask and a diffraction grating on a wafer. A diffraction order is detected at a predetermined angle and the phase and amplitude of a known frequency component of the intensity determined to obtain alignment information about the mask and wafer.
All of the above alignment devices or sensors have performed adequately for aligning a wafer and mask. While alignment marks on the mask or reticle and the wafer have been used with acceptable results, the ever decreasing feature size or line width being produced on the wafer results in the need for improved alignment sensors. The difficulty of accurate alignment is increased when various processing steps result in multiple layers being formed on the wafer that coat or obscure the alignment marks making their detection difficult with conventional alignment sensors. Producing alignment marks on the mask or reticle of high quality also undesirably adds to the cost of the reticle or mask, and therefore, the overall manufacturing process. There is a need for an improved alignment sensor and alignment method that will improve the alignment between a mask and a wafer and provide better reliability independent of process steps in a variety of applications. Additionally, there is a continuing need to improve alignment sensors in both accuracy and speed, as semiconductor manufacturing technologies advance.
The present invention uses an interferometer arrangement to determine mask and wafer alignment. An illumination source provides a coherent electromagnetic radiation, which may have multiple discreet wavelengths, to a beamsplitter. The beamsplitter divides the coherent electromagnetic radiation to illuminate at near normal incidence a first fixed or stationary diffraction grating and a second diffraction grating placed on a movable wafer. The diffracted orders from the fixed reference grating are collected, together with the diffracted orders from the movable wafer grating. The motion of the movable wafer grating causes a measurable phase shift. The collected diffraction orders are detected and the phase shift determined. A signal processor calculates any misalignment based on the phase shift and other information, and provides control signals to a stage controlor. Alignment is therefore maintained between the wafer grating and the fixed grating. By other means a mask stage is accurately positioned with respect to the fixed grating. The use of multiple channels containing different diffraction orders and different wavelengths or colors of electromagnetic radiation from the illumination source helps to attain alignment information irrespective of processing variables associated with different layers or coatings on a wafer. By selectively polarizing the illumination source and providing a central polarized portion on the beamsplitter, the different channels or diffraction orders and wavelengths can be balanced for optimum contrast. Alternatively, the contrast can be minimized, suppressing the interference, allowing the alignment sensor to operate in a mode to permit the use of latent image metrology (LIM) methods, by measuring diffracted intensity.
Accordingly, it is an object of the present invention to improve alignment of a wafer and a reticle irrespective of processing variables.
It is a further object of the present invention to provide redundant alignment information.
It is an advantage of the present invention that the phase detection required for determining alignment is relatively simple.
It is a further advantage of the present invention that the expense of providing a grating on a mask or reticle is avoided.
It is a feature of the present invention that it has an interferometer geometry.
It is another feature of the present invention that a fixed grating is used.
It is yet another feature of the present invention that multiple discreet wavelengths may be used to obtain necessary alignment information, irrespective of processing variables.
These and other objects, advantages, and features will become readily apparent in view of the following more detailed description.